This invention relates to a gas valve particularly for use in association with a gas filter of a kind which is periodically flushed, i.e., cleaned, by providing a reverse flow pulse of gas through it.
Such filters are known in the field of gas turbines where the working fluid includes combustion products at high temperatures and it is vital to exclude combustion particles since they can seriously damage turbine blading. So-called candle filter elements are commonly employed, these each consisting of a porous ceramic tube closed at one end which projects into the `dirty` gas zone. The dirty gas is driven through the filter by a pressure differential leaving the dirt adhering (largely) to the filter surface.
It is known in such systems to reverse flush the filter by applying a pulse of high pressure gas to the inside of the filter element. The dirt is blown off the filter surface and falls to the bottom of the container or pressure vessel where it is collected and disposed of.
The reverse pulse of gas is obtained from the clean gas side of the filter. It is cooled then compressed to a very high pressure and reheated and stored in a reservoir.
The gas pulse is then passed through an external pulsing valve to a nozzle at the entrance to a venturi at the neck of the outlet side of the filter element holder.
The reverse flush valve is opened and shut very quickly (usually under 1 second) and a pulse of gas is fed to the nozzles. One valve can normally supply 10 nozzles.
The pulse of gas leaves the nozzle and enters the venturi where it entrains clean gas and after going through the venturi enters the bore of the ceramic element.
The pulse wave generated then causes reverse flow through the walls of the element and produces a shock wave on the caked dirt sufficient to break it away ready for collection at the bottom of the vessel.
The intensity of the pulse inside the element is, however, minute compared to the initial pressure in the reservoir and the system is very inefficient.
A further problem that arises in such systems is that, in the event of the filter element breaking off, dirty gas is allowed to pass directly through the venturi and to enter the gas turbine. This dirt has a very detrimental effect on the fuel control valves and the turbine blading.